Z3 M Coupe Rear Subframe Reinforcement: Part 1

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Z3 M Coupe Rear Subframe Reinforcement: Part 1

It is well known that the Z3 chassis, especially the M Coupe and Roadsters, suffer from rear trunk floor issues. While BMW produced arguably their best vehicles in the 90s and early 00s, many of the models suffered from rear subframe and trunk floor issues. Namely the E36, E46, and of course, the E36/7 and E36/8 chassis.

During the mini-restoration of our M Coupe we decided it was best to check the rear trunk floor for any damage. Looking underneath, there was a small crack in the differential mounting ear. We checked inside of the hatch area and removed the sound deadening revealing a small separation of the trunk floor panel on the driver’s side.

 

 

We knew this had to be addressed appropriately to prevent further failure of the rear chassis. We immediately placed an order with the renowned guru for the E36/7 and E36/8 chassis, Randy Forbes at spcarsplus.com, for a reinforcement kit. We sourced a new chassis differential ear mount from BMW as per Randy Forbes’ recommendation for a single ear reinforcement as we didn’t want to deviate from stock form and go with a dual ear setup.

 

Parts and tools used:

  • Hobart 140 welder
  • electric cut off tool with 1mm cutting discs
  • 1/2 inch belt grinder with 40, 60 and 100 grit belts
  • spot weld drill bit
  • various clamps
  • Ingersoll Rand 90 degree angle grinder with Roloc 50 grit discs
  • BMW differential bracket – (part 41118398664)

 

Step 1: Remove the sound deadening.

BMW uses a tar-like mat for sound deadening. There are numerous posts across the web of individuals removing this on many different models of BMWs in order to save weight. Some use dry ice successfully to cause the mat to lift up in nearly one piece, some just chisel away, and others use heat.

We chose to use a $20 Wagner heat gun from Lowe’s and a putty knife. This worked very well, but left some residue after the mat was lifted. To our surprise, the stuff is pretty hefty.

Interestingly, you can see where the factory ‘imperfectly’ installed the sound deadening on the left by just placing it and tearing part of it away for clearance around the stud.

 

All of the necessary sound deadening removed with the seam sealer on the frame rails next to do. We followed many of Randy Forbes’ photos of his installations and noticed he always stops 3/4 up on the left side and leaves the factory sound deadening. We attempted this but ended up removing the rest of it before installing new sound deadening. This resulted in a much cleaner install and look in the end. Our recommendation is to remove all sound deadening below the front mounts to begin with.

Seam sealer on the frame rails removed along with the bottom corners of the sound deadening against the bulk head. The seam sealer needs to be heated, as well. It becomes gooey – you just need to keep scraping.

What was removed was a decent chunk of weight.

After the seam sealer was removed, we were able to inspect the spot welds. Here is one that had started to pull through. If left this way, it would have resulted in further degradation and separation of the rear trunk floor on the driver’s side.

Step 2: Prepare for surgery.

Next, we cleaned up the sound deadening residue and began prepping for cutting into the floor. To do this, we got some shop rags and old gasoline (the cheapest solvent) for clean-up. Since the sound deadening is tar-like and fossil fuel based, it reacted well to the gasoline and allowed for somewhat easy removal with some scrubbing.

The first step was to remove the dead floor brackets. These brackets have to be put back in place after the reinforcement is done so it is important to not damage them. This can be done by using a spot weld drill bit to drill out the spot welds of the brackets.

On each bracket one spot weld was drilled too deep, but it was easy enough to put the piece of metal back and weld in place when finished.

After the driver’s side bracket was removed.

After the passenger side bracket was removed.

After both brackets are removed this is what is left. Sharpie marks roughly where the metal needed to be cut.

Then the point of no return cutting began. A welding blanket was clipped up inside of the rear hatch to keep sparks from flying around during cutting and welding.

A cheap cut off saw bought off of Amazon was used with 1mm cutting discs to cut through the metal.

To be able to keep cutting in a straight line one of the welded studs had to be removed as pictured below.

A pair of vice grips was used to clamp on to the stud and bend back and forth a couple times to break it loose.

The access panel was then completely cut out.

The rough opening revealed the majority of the remnants from the cutting process along with some long past treats from some little critters.

A Dremel with a cutting disc was used to cut excess areas of metal to make the trunk floor as even as possible with the inner cavity.

After vacuuming out the cavity an Ingersoll Rand air powered 90 degree angle grinder with 3 inch Roloc discs was used to grind the edge smooth and clean up any areas of rough metal.

After the edges were cut and smoothed out the three raised bumps of the trunk floor had to be flattened out. This was done by cutting slits on either side of the raised bumps. They were then bent up, paint ground off and hammered back down to make them flat with the trunk floor.

This is the bump on the back side of the trunk floor that had to be flattened as well.

 

After the raised bumps of the trunk floor were prepared and hammered flat.

After preparing the trunk floor we decided to proceed forward with reinforcing the single diff ear on the M Coupe subframe. The first thing to do was to prepare the differential ear bracket bought from FCPEuro for reinforcing the existing differential ear on the M Coupe.

A handheld spring punch was used to mark several areas where holes were drilled. These holes were used to plug weld the reinforcement on to the existing bracket.

It took some assistance to drill the holes as the bracket is made from a heavy gauge metal.

Before welding the reinforcement for the diff ear the small crack in the diff ear was stop-drilled to prevent any possible spreading.

It was then sanded and welded up.

The reinforcement plate was cut into three pieces and test fit to the existing diff ear. Then both the diff ear and reinforcement were ground to bare metal for welding.

A 1/2 inch belt sander came in handy for a lot of the small areas underneath the trunk floor including removing paint and grinding down welds.

Step 3: Welding the single diff ear reinforcement.

The reinforcement was then fitted up to the differential ear and clamped into place with multiple welding clamps. The bolt was also used to help align it as well.

Welding commenced on the rear diff ear reinforcement with plug welding.

A look at the inside cavity to check on weld penetration.

After the plug welds were complete the edges of the reinforcement were then stitch welded.

The edges were welded in small sections and alternated from side to side.

The back side of the diff ear mount was bent down in preparation of the other reinforcement to be welded on.

The mount was then sanded to bare metal and the new reinforcement was clamped on with welding clamps.

After the plug welds were completed, the stock diff ear mount was hammered back flat with the new reinforcement.

Reinforcement plugs welds completed and sanded on all sides of diff mount.

Part 2 continues with the reinforcement of the rear trunk floor.

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